1. Field of the Invention
The present invention relates a communication apparatus, communication system, communication method, and communication control program which perform media access control (MAC) and, more particularly, to frame aggregation for transmitting a plurality of media access control frames (MAC frames) upon containing them in one physical frame (PHY frame).
2. Description of the Related Art
Media access control (MAC) is control for causing a plurality of communication apparatuses which perform communication while sharing the same medium to decide how to use the medium in transmitting communication data or management frame. Owing to media access control, even if two or more communication apparatuses transmit communication data (or management frame) by using the same medium at the same time, there is less chance of the occurrence of a phenomenon (collision) in which a communication apparatus on the receiving side cannot decode communication data. Media access control is also a technique for controlling access from communication apparatuses to a medium so as to minimize the chance of the occurrence of a phenomenon in which, despite the presence of communication apparatuses having transmission requests, the medium is not used by any of the communication apparatuses.
In radio communication, since it is difficult for a communication apparatus to monitor transmission data while transmitting the data, media access control which is not premised on collision detection is required. IEEE 802.11, which is a typical technical standard for wireless LANs, uses CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). According to CSMA/CA in IEEE 802.11, the MAC header has the duration value which is the time, in microseconds, required to transmit the data or management frame (also including SIFS interval). In this duration, a communication apparatus which is irrelevant to the sequence and has no transmission right waits for transmission upon determining a virtual busy state of the wireless medium. This prevents the occurrence of collision. The CSMA/CA is designed to reduce the collision probability. IEEE 802.11 defines that the state of a medium is determined on the basis of such a combination of virtual carrier sense on a MAC layer and physical carrier sense on a physical layer (PHY layer), and media access control is performed on the basis of the determination.
IEEE 802.11 using CSMA/CA has increased the communication speed mainly by changing the physical layer protocol. With regard to the 2.4 GHz band, there have been changes from IEEE 802.11 (established in 1997, 2 Mbps) to IEEE 802.11b (established in 1999, 11 Mbps), and further to IEEE 802.11g (established in 2003, 54 Mbps). With regard to the 5 GHz band, only IEEE 802.11a (established in 1999, 54 Mbps) exists as a standard. In order to develop standard specifications directed to further increase communication speeds in both the 2.4 GHz band and the 5 GHz band, IEEE 802.11 TGn (Task Group n) has already been established.
Even if an attempt to increase the communication speed in terms of physical layer succeeds, the effective throughput of communication cannot be improved. That is, when an increase in the communication speed of the physical layer is realized, the format of a PHY frame (PHY header and PHY preamble) ceases to be effective any more. An increase in overhead due to this may hinder an increase in throughput. In a PHY frame, a temporal parameter associated with CSMA/CA is permanently attached to a MAC frame. In addition, a PHY frame header is required for each MAC frame.
As a method of solving the problem of overhead and increasing throughput, a block response (Block acknowledgement) mechanism introduced in recently drafted IEEE 802.11e/draft 5.0 (enhancement of QoS in IEEE 802.11) is available. The block response mechanism can consecutively transmit a plurality of MAC frames without any random backoff (with SIFS interval), and hence can reduce the backoff amount to some degree. However, the overhead of a physical layer header and preamble cannot be effectively reduced. In addition, according to aggregation introduced in initially drafted IEEE 802.11e, both the backoff amount and the physical layer header can be reduced. However, since the length of a physical layer frame containing MAC frames cannot be increased beyond about 4 kbytes under the conventional limitation on the physical layer, an improvement in efficiency is greatly limited. Even if the length of a PHY layer frame can be increased, another problem arises, i.e., a reduction in error tolerance.